1. Field of the Invention
The present invention relates to a liquid crystal display device that produces a display by utilizing light reflected from a reflective layer, and more particularly to a liquid crystal display device wherein both white and black display colors are achromatic colors, and wherein reflectivity in white display state is high and reflectively in black display state is low.
2. Description of the Related Art
Liquid crystal displays, which are thin and light compared with other types of display, are widely used in various applications including displays for portable information terminals. A liquid crystal display contains a liquid crystal cell as a light receiving type display element. The liquid crystal cell does not emit light by itself, but produces a display by changing its light-transmitting properties by being driven with an operating voltage of 1 to 9 volts. Accordingly, a reflective mode liquid crystal display, which displays images by reflecting ambient light using a reflector mounted underneath the liquid crystal, is an extremely low power consumption display device. It is also known that if a super twisted nematic (STN) liquid crystal cell is used in the liquid crystal display device, the price of the liquid crystal display device can be reduced because the construction of the liquid crystal display device can be simplified.
Conventionally, STN reflective liquid crystal displays utilize optical birefringence, whereas twisted nematic (TN) reflective liquid crystal displays utilize optical rotatory power. In STN reflective liquid crystal displays, therefore, optical compensation is difficult since the polarization state of the emergent light varies depending on the amount of birefringence in the liquid crystal layer. As shown in FIG. 9, a prior art STN reflective liquid crystal display device 1 comprises a liquid crystal cell 3 sandwiched between two polarizers 4 and 5 to enhance optical compensation. In the liquid crystal display device 1 as shown in FIG. 9, a phase retardation plate 6 is interposed between the liquid crystal cell 3 and the first polarizer 4. The second polarizer 5 is located between a reflector 7 and the liquid crystal cell 3. The liquid crystal cell 3 is constructed by sandwiching a liquid crystal layer 8 between two transparent substrates 9 and 10. Each substrate 9, 10 comprises an alignment film and electrodes formed on a substrate base.
When constructed as a color display using color filters, the reflective liquid crystal display device 1 which uses two polarizers involves the problem that optical reflectivity enough to provide sufficient brightness for display is difficult to obtain because of the light loss associated with the color filters. Furthermore, in the reflective liquid crystal display device 1, since the two polarizers 4 and 5 are mounted on the outside of the liquid crystal cell 3, the reflector 7 also has to be mounted on the outside of the liquid crystal cell 3. As a result, light loss is caused due to the presence of the substrate 10 of the liquid crystal cell 3 on the second polarizer 5 side.
In the reflective liquid crystal display device 1 using two polarizers, the substrate 10 about 1 mm thick and the second polarizer 5 about 0.2 mm thick are interposed between the reflector 7 and the liquid crystal layer 8. Consequently, when light is incident obliquely on the reflective liquid crystal display device 1 of FIG. 9, as shown in FIG. 10, the incident light when reflected passes through a pixel different from the pixel it passed through when it entered the cell. In this case, if the reflective liquid crystal display device 1 is viewed perspectively, it will be seen that parallax is caused such that the shade 16 of a displayed object appears to be projected on the reflector 7. Poor viewability caused by such parallax is also a problem with the reflective liquid crystal display device 1 of FIG. 9.
In view of the above situation, there is proposed, as shown in FIG. 11, a reflective liquid crystal display device 11 in which one polarizer is omitted to achieve an improvement in brightness corresponding to the omission of one polarizer. In FIG. 11, the same constituent elements as those in FIG. 9 are designated by the same reference numerals and an explanatory description thereof will not be given here. In the reflective liquid crystal display device of FIG. 11, only one polarizer 4 is arranged on the side of the liquid crystal cell 3 opposite from the reflector, and the second polarizer 5 is omitted. The applicant further proposes in Japanese Unexamined Patent Publication JP-A 7-84252 (1995) a reflective liquid crystal display device having only one polarizer and characterized by the provision of a reflector 7 within the liquid crystal cell 3. The reflective liquid crystal display device 11 having the structure as shown in FIG. 11 can thus eliminate the problem of light loss caused by the presence of the substrate 10 of the liquid crystal cell 3 on the second polarizer 5 side. Reflective liquid crystal display devices of the type in which the reflector 7 is disposed within the liquid crystal cell 3 are also disclosed in Japanese Unexamined Patent Publications JP-A 10-161110 (1998) and JP-A 10-170906 (1998). In the reflective liquid crystal display device of FIG. 11, the problem of poor viewability caused by parallax is also solved because of the absence of the substrate base and the second polarizer 5 between the liquid crystal layer 8 and the reflector 7.
FIG. 12A is a schematic diagram for explaining how the light loss occurs in the reflective liquid crystal display device 1 of FIG. 9 that has two polarizers. FIG. 12B is a schematic diagram for explaining how the light loss is reduced in the reflective liquid crystal display device 11 of FIG. 11 that has a single polarizer. The explanation of FIGS. 12A and 12B is given assuming that the transmittance of light per polarizer is 45% and that the transmittance of the polarization component parallel to the absorption axis of the polarizer is 0%. Further, in the explanation of FIGS. 12A and 12B, light absorption into color filters is not considered. In the examples of FIGS. 12A and 12B, of the polarization component orthogonal to the absorption axis of the polarizer, which accounts for 50% of the incident light, 10% is absorbed by the polarizer; this means that the transmittance of the polarization component orthogonal to the absorption axis, per polarizer, is 90%.
In the reflective liquid crystal display device 1 of FIG. 9 that uses two polarizers, since the incident light on the device 1 emerges from it after passing the polarizers a total of four times, the reflectivity is 32.8% as shown by expression (1). On the other hand, in the reflective liquid crystal display device 11 of FIG. 11 in which only one polarizer is used and the reflector is disposed within the liquid crystal cell 3, the incident light on the device 11 emerges from it after passing through the polarizer two times; therefore, the reflectivity is 40.5% as shown by expression (2). As can be seen from the above results, the reflective liquid crystal display device 11 having only one polarizer has the potential of providing up to about 23.5% improvement in reflectivity over the reflective liquid crystal display device 1 having two polarizers.
Reflectivity of liquid crystal display device of FIG. 9=0.9xc3x970.9xc3x970.9xc3x970.9xc3x9750%=32.8%xe2x80x83xe2x80x83(1)
Reflectivity of liquid crystal display device of FIG. 11=0.9xc3x970.9xc3x9750%=40.5%xe2x80x83xe2x80x83(2)
In the liquid crystal display device 11 having a single polarizer and a single reflector, however, the omission of one polarizer makes optical compensation all the more difficult, and the display background color which should be white or black is caused to shift. Specifically, in the case of an STN liquid crystal cell that utilizes optical birefringence, the color shift becomes pronounced, compared with the case of a TN liquid crystal cell that utilizes optical rotatory power. In the reflective liquid crystal display device 11 that uses an STN liquid crystal cell and has a single polarizer, color shifting is a major problem that must be overcome.
Japanese Unexamined Patent Publication JP-A 4-97121 (1992) discloses a technique for solving the color shift problem in a reflective liquid crystal display device having a single polarizer. The reflective liquid crystal display device disclosed in JP-A 4-97121 includes, in addition to the liquid crystal forming the liquid crystal cell, at least one optically anisotropic layer in order to eliminate the color shift occurring when the display device is operated in a reflective STN mode. The optically anisotropic layer is realized using a uniaxially oriented polymer film, and functions as a phase retardation layer. When increasing the viewing angle in the horizontal direction of the liquid crystal display device, the rubbing direction of the liquid crystal cell is determined so that the orientation direction of the uniaxially oriented film, that is, the retardation axis of the phase retardation layer, can be arranged parallel to the horizontal direction of the display screen. When increasing the contrast ratio of the liquid crystal display device, the rubbing direction is determined so that the retardation axis can be arranged parallel to the vertical direction of the display screen. In JP-A 4-97121, the combination of the angle that the absorption axis of the polarizer makes with the retardation axis of the phase retardation plate and the retardation values of the liquid crystal layer and the phase retardation layer, respectively, is (18xc2x0, 810 nm, 310 nm), (11xc2x0, 730 nm, 370 nm), or (75xc2x0, 690 nm, 360 nm).
The applicant also discloses in JP-A 7-84252 a technique for achromatizing the display color by eliminating the color shift occurring when displaying intermediate tones on a reflective liquid crystal display device having a single polarizer. To prevent the color shift caused by the birefringence properties of the liquid crystal, the reflective liquid crystal display device disclosed in JP-A 7-84252 includes, in addition to the liquid crystal forming the liquid crystal cell, an optical phase compensation member which is a phase retardation plate formed from at least one optically anisotropic layer. The retardation value of the liquid crystal, the retardation value of the optical phase compensation member, the direction of the polarization axis of the polarizer, the direction of the retardation axis of the optical phase compensation member, and the orientation direction of liquid crystal molecules are optimized to prevent the color shift from occurring when displaying intermediate tones. In JP-A 7-84252, the combination of the angle that the absorption axis of the polarizer makes with the retardation axis of the phase retardation plate and the retardation values of the liquid crystal layer and the phase retardation layer, respectively, is (85xc2x0, 650 nm, 350 nm) or (25xc2x0, 650 nm, 350 nm).
For the optimization of the reflective liquid crystal display device 11 of the type that uses a single polarizer and a phase retardation plate, the following optimization theory is generally known. When the difference between the retardation value dxcex94n of the liquid crystal layer and the retardation value of the phase retardation plate is approximately an integral multiple of one-quarter wavelength for all wavelengths of visible light, then the optical path difference when the light makes a round trip between the polarizer and the reflector is approximately an integral multiple of one-half wavelength for every wavelength of the visible light. Thus the light is ideally blocked by or transmitted through the polarizer. Accordingly, the reflective liquid crystal display device that uses the phase retardation plate and the polarizer respectively having the above defined retardation values can produce an ideal dark display state of low reflectivity or an ideal bright display state of high reflectivity.
The above optimization theory for reflective liquid crystal displays has previously be known for the case where the liquid crystal molecules in the liquid crystal cell are homogeneously aligned, as described in Japanese Unexamined Patent Publication JP-A 6-337414 (1994). In the liquid crystal display device disclosed in JP-A 6-337414, the liquid crystal molecules in the liquid crystal cell are aligned to provide a TN liquid crystal molecular alignment with a twist angle of 180xc2x0 to 270xc2x0, that is, an STN alignment, in order to eliminate the viewing angle dependence of the reflective liquid crystal display device optimized based on the above optimization theory. In the liquid crystal display device disclosed in JP-A 6-337414, the combination of the angle that the absorption axis of the polarizer makes with the retardation axis of the phase retardation plate and the retardation values of the liquid crystal layer and the phase retardation layer, respectively, is (0xc2x0, 1090 nm, 320 nm) or (90xc2x0, 1090 nm, 320 nm); that is, the absorption axis and the retardation axis are parallel or orthogonal to each other.
The applicant also proposes in Japanese Unexamined Patent Publication JP-A 7-146469 (1995) a reflective liquid crystal display device having a single polarizer and a phase retardation plate based on the above optimization theory. In the liquid crystal display device disclosed in JP-A 7-146469, the polarizer is arranged on one side of the liquid crystal cell and the reflector on the opposite side of the liquid crystal cell, and a quarter wave plate as the phase retardation plate is interposed between the reflector and the opposite side of the liquid crystal cell. In this liquid crystal display device, the retardation value of the liquid crystal, the direction of the polarization axis of the polarizer, the direction of the retardation axis of the quarter wave plate, and the direction of the long axis of the liquid crystal molecules are optimized to eliminate color shifting and improve the contrast at the same time. This liquid crystal display device produces a white display when a voltage is applied across the liquid crystal layer so that the amount of birefringence in the liquid crystal layer becomes approximately equal to one-quarter wavelength, and a black display when a voltage is applied across the liquid crystal layer so that the amount of birefringence in the liquid crystal layer becomes approximately equal to zero.
Japanese Unexamined Patent Publication JP-A 10-123505 (1998) discloses a technique for preventing the color shift caused by the wavelength dependence of the amount of birefringence in the liquid crystal layer in a reflective liquid crystal display device designed based on the optimization theory. The liquid crystal display device disclosed in JP-A 10-123505 comprises a polarizer arranged on one side of the liquid crystal cell, a reflector placed between the liquid crystal layer and the substrate on the opposite side of the liquid crystal cell, and a compensation plate as a phase retardation plate, interposed between the polarizer and the one side of the liquid crystal cell, for the optical compensation of the liquid crystal layer. The compensation plate is constructed so that the wavelength dependence of the amount of birefringence in the compensation plate matches the wavelength dependence of the amount of birefringence in the liquid crystal layer, and the wavelength dependence of the amount of birefringence in the liquid crystal layer is canceled by the compensation layer. Such a compensation plate is realized using a uniaxially oriented film. In JP-A 10-123505, the angle that the absorption axis of the polarizer makes with the retardation axis of the compensation plate is 45xc2x0 or 135xc2x0.
As described above, the reflective liquid crystal display device using a single polarizer utilizes a phase retardation plate formed from a single uniaxially oriented film to prevent color shifting. The compensation effect for color shift prevention that can be obtained by one uniaxially oriented film is smaller than the compensation effect required to eliminate the color shift in a liquid crystal display. Japanese Unexamined Patent Publications JP-A 10-170906 (1998), JP-A 10-232390 (1998), JP-A 9-292610 (1997), and JP-A 9-43596 (1997) disclose reflective liquid crystal display devices that use two or more phase retardation plates for the prevention of color shifting. These two or more phase retardation plates are each constructed from a single phase retardation layer, and differ from phase retardation plates of the type constructed by stacking a plurality of phase retardation layers one on top of another and combining these layers into a single plate. When using two or more phase retardation plates, the overall cost of the liquid crystal display device increases as the number of phase retardation plates used increases. Accordingly, liquid crystal display devices of the type that uses a single phase retardation plate can save manufacturing costs and are therefore advantageous in terms of cost compared with the type that uses two or more phase retardation plates.
For the reason described above, liquid crystal display devices of the type that uses a single phase retardation plate and a single polarizer are receiving attention nowadays, and intense research efforts are underway aiming at further optimization of liquid crystal display devices of the type that uses a single polarizer and a single phase retardation plate. However, in the case of liquid crystal display devices constructed using a single polarizer and a single phase retardation plate, particularly, liquid crystal display devices that uses an STN liquid crystal cell, optical compensation for the wavelength dependence of the amount of birefringence becomes difficult when the single phase retardation plate is formed from a single liquid crystal layer. For this reason, any of the liquid crystal display devices disclosed in the above-cited patent publications does not reach satisfactory levels for practical use in terms of the brightness of the white display, the contrast ratio, and the achromatization of white and black display colors.
An object of the invention is to provide a liquid crystal display device comprising a single polarization layer and a single phase retardation layer and utilizing reflected light, capable of achieving a bright white display and high contrast, while achieving achromatization of white and black display colors.
The invention provides a liquid crystal display device comprising:
a single polarization layer for transmitting therethrough only a linearly polarized component of incident light, which is polarized in a predetermined direction;
a reflective layer for reflecting light;
a single phase retardation layer disposed between the polarization layer and the reflective layer; and
a liquid crystal layer disposed between the polarization layer and the reflective layer,
wherein retardation value ReF of the phase retardation layer is selected to be approximately equal to a (1/4+K/2)th multiple of wavelength xcex of the incident light (where K is an integer not smaller than 0);
product d1xc3x97xcex94n1 of thickness d1 and optical anisotropy xcex94n1 of the liquid crystal layer is selected to be approximately equal to a (1/2+L/2)th multiple of the wavelength xcex of the incident light (where L is an integer not smaller than 0); and
crossing angle xcex94xcfx86 between an absorption axis of the polarization layer and a retardation axis of the phase retardation layer is selected to be larger than 0xc2x0 and smaller than 45xc2x0, or larger than 45xc2x0 and smaller than 90xc2x0.
According to the invention, the liquid crystal display device includes a single polarization layer and a single phase retardation layer in addition to the liquid crystal layer and the reflective layer. The optical properties of the portion of the liquid crystal display device that comprises the liquid crystal layer and the phase retardation layer are adjusted so that linearly polarized light is changed to a state close to circular polarization when the wavelength of the incident light is 550 nm.
The thus constructed liquid crystal display device produces a display by utilizing the light reflected from the reflective layer, while correcting for the effects of optical birefringence. In this liquid crystal display device, the retardation value ReF of the phase retardation layer is set approximately equal to (1/4+K/2), the product d1xc3x97xcex94n1 of the thickness d1 and optical anisotropy xcex94n1 of the liquid crystal layer is set approximately equal to (1/2+L/2)xcex, and the crossing angle xcex94xcfx86 between the absorption axis of the polarization layer and the retardation axis of the phase retardation layer is set as 0xc2x0 less than xcex94xcfx86 less than 90xc2x0 (where xcex94xcfx86xe2x89xa045xc2x0). With this arrangement, the polarization state of the incident light, when passing through the phase retardation layer, is changed to a state slightly displaced from circular polarization so that the light of optimum polarization state reaches the liquid crystal layer side surface of the reflective layer. The reason for xcex94xcfx86xe2x89xa045xc2x0 is that if xcex94xcfx86=45xc2x0, the polarization state of the light passed through the phase retardation layer is circular polarization, in which case the polarization state after passing through the liquid crystal layer will deviate from the optimum state (ideally, circular polarization) due to the effects of birefringence. Thus, compared with the prior art liquid crystal display devices having a single polarization layer and a single phase retardation layer, the liquid crystal display device of the invention achieves the characteristics that can increase the brightness of white display, improve the contrast, and render the white and black display states in achromatic colors.
As described above, according to the invention, the liquid crystal display device includes a single polarization layer and a single phase retardation layer in addition to the liquid crystal layer and the reflective layer, and the portion comprising the liquid crystal layer and the phase retardation layer has the optical properties such that linearly polarized light passing therethrough is converted to a state close to circular polarization. Thus the liquid crystal display device achieves the characteristics that increase the brightness of white display, improve the contrast, and render the white and black display states in achromatic colors. To achieve such characteristics, in the liquid crystal display device, the retardation value ReF of the phase retardation layer is set approximately equal to (1/4+K/2)xcex, the product d1xc3x97xcex94n1 of the thickness d1 and optical anisotropy xcex94n1 of the liquid crystal layer is set approximately equal to (1/2+L/2)xcex, and the crossing angle xcex94xcfx86 between the absorption axis of the polarization layer and the retardation axis of the phase retardation layer is set as 0xc2x0 less than xcex94xcfx86 less than 90xc2x0 (where xcex94xcfx86xe2x89xa045xc2x0). With this arrangement, the liquid crystal display device can reliably achieve the characteristics that increase the brightness of white display, improve the contrast, and render the white and black display states in achromatic colors.
In the liquid crystal display device of the invention it is preferable that the phase retardation layer is disposed between the liquid crystal layer and the polarization layer, and the incident light passed through the polarization layer and the phase retardation layer is in a state of elliptical polarization which is close to circular polarization.
According to the invention, the amount of displacement from circular polarization, applied in the portion comprising the polarization layer and the retardation layer in the liquid crystal display device, is relatively small, and the incident light passed through the polarization layer and then through the retardation layer emerges as elliptically polarized light, the polarization state slightly displaced from circular polarization. The light used for display can thus be set to a further optimized state just before it reaches the liquid crystal layer side of the reflective layer. Accordingly, the liquid crystal display device can further improve the contrast and enhance the achromatization.
In the liquid crystal display device of the invention, it is preferable that the crossing angle xcex94xcfx86 between the absorption axis of the polarization layer and the retardation axis of the phase retardation layer is selected to fall within a range of 40xc2x0xc2x13xc2x0.
According to the invention, in the liquid crystal display device, the crossing angle xcex94xcfx86 between the absorption axis and the retardation axis is set at approximately 40xc2x0. With this arrangement, the polarization state of the light that strikes the reflective layer on the surface thereof closest to the liquid crystal layer can be further optimized, and thus the liquid crystal display device can provide an optimum high contrast ratio and achieve optimum achromatization. The reason for setting the tolerance of xc2x13xc2x0 is that the optimum value somewhat varies depending on the retardation value of the liquid crystal layer, the retardation value of the phase retardation layer, and the crossing angle between the orientation axis of liquid crystal molecules on the bottom substrate and the absorption axis of the polarization layer.
In the liquid crystal display device of the invention, it is preferable that the liquid crystal layer is formed as a super twisted nematic layer, the twist angle of liquid crystal molecules in the liquid crystal layer is selected to be 180xc2x0 or larger, and the product d1xc3x97xcex94n1 of the thickness d1 and optical anisotropy xcex94n1 of the liquid crystal layer, in the case where the wavelength xcex of the incident light is 550 nm, is selected to be larger than 760 nm and smaller than 860 nm.
According to the invention, the liquid crystal display device is constructed as an STN liquid crystal display device that produces a display by utilizing birefringence. The liquid crystal layer is designed so that the product d1xc3x97xcex94n1 of the thickness d1 and optical anisotropy xcex94n1 of the liquid crystal layer is larger than 760 nm but smaller than 860 nm for the incident light wavelength of 550 nm where the visual perception is the highest. The reason for this is as follows. If the product d1xc3x97xcex94n1 is 760 nm or smaller or 860 nm or larger when the incident light wavelength is 550 nm, the product d1xc3x97xcex94n1 deviates widely from an integral multiple of one-half wavelength. As a result, the same effect as provided by a half wave plate becomes difficult to obtain with the liquid crystal layer, and therefore, the contrast of the liquid crystal display device drops below the practical level. If the product d1xc3x97xcex94n1 is larger than 760 nm and smaller 860 nm when the incident light wavelength is 550 nm, the product d1xc3x97xcex94n1 becomes approximately equal to an integral multiple of one-half wavelength; as a result, the liquid crystal display device can provide sufficient contrast for practical use. The reason for setting tolerance for the retardation value of the liquid crystal layer is that the optimum value somewhat varies depending on the crossing angle xcex94xcfx86 between the absorption axis of the polarization layer and the retardation axis of the phase retardation layer, the retardation value of the phase retardation layer, and the crossing angle between the orientation axis of liquid crystal molecules on the bottom substrate and the absorption axis of the polarization layer.
In the liquid crystal display device of the invention, it is preferable that the liquid crystal layer is formed as a super twisted nematic layer, the twist angle of liquid crystal molecules in the liquid crystal layer is selected to be 180xc2x0 or more, and the product d1xc3x97xcex94n1 of the thickness d1 and optical anisotropy xcex94n1 of the liquid crystal layer, when the wavelength xcex of the incident light is 550 nm, is selected to fall within a range of 770 nm to 856 nm.
According to the invention, the liquid crystal display device is constructed as an STN type liquid crystal display device. The liquid crystal layer is designed so that the product d1xc3x97xcex94n1 of the thickness d1 and optical anisotropy xcex94n1 of the liquid crystal layer is not smaller than 770 nm but not larger than 856 nm for the incident light wavelength of 550 nm. With this arrangement, the liquid crystal display device can achieve a contrast of 2 or higher, providing optimum display quality for practical use. If the product is smaller than 770 nm or larger than 856 nm, the contrast will further decrease.
In the liquid crystal display device of the invention, it is preferable that the liquid crystal layer is formed as a super twisted nematic layer, the twist angle of liquid crystal molecules in the liquid crystal layer is selected to be 180xc2x0 or larger, and the retardation value ReF of the phase retardation layer, when the wavelength xcex of the incident light is 550 nm, is selected to be larger than 360 nm and smaller than 450 nm.
According to the invention, the liquid crystal display device is constructed as an STN liquid crystal display device that produces a display by utilizing birefringence. The phase retardation layer is formed so that the retardation value ReF of the phase retardation layer is larger than 360 nm but smaller than 450 nm for the incident light wavelength of 550 nm where the visual perception is the highest. The reason for this is as follows. If the retardation value of the phase retardation layer is 360 nm or smaller or 450 nm or larger when the incident light wavelength is 550 nm at which the visual perception is the highest, the retardation value of the phase retardation layer deviates widely from a (2L+1)th multiple of one-quarter wavelength; as a result, the contrast of the liquid crystal display device drops below the practical level. If the retardation value of the phase retardation layer is larger than 360 nm and smaller 450 nm when the incident light wavelength is 550 nm, the retardation value of the phase retardation layer becomes approximately equal to a (2L+1)th multiple of one-quarter wavelength; as a result, the liquid crystal display device can provide sufficient contrast for practical use. The reason for setting tolerance for the retardation value of the phase retardation layer is that the optimum value somewhat varies depending on the crossing angle xcex94xcfx86 between the absorption axis of the polarization layer and the retardation axis of the phase retardation layer, the retardation value of the liquid crystal layer, and the crossing angle between the orientation axis of liquid crystal molecules on the bottom substrate and the absorption axis of the polarization layer.
In the liquid crystal display device of the invention, it is preferable that the liquid crystal layer is formed as a super twisted nematic layer, the twist angle of liquid crystal molecules in the liquid crystal layer is selected to be 180xc2x0 or larger, and the retardation value ReF of the phase retardation layer, when the wavelength xcex of the incident light is 550 nm, is selected to fall within a range of 365 nm to 445 nm.
According to the invention, the liquid crystal display device is constructed as an STN liquid crystal display device. The phase retardation layer is designed so that the retardation value ReF of the phase retardation layer falls within a range of 365 nm to 445 nm for the incident light wavelength of 550 nm where the visual perception is the highest. With this arrangement, the liquid crystal display device can achieve a contrast of 2 or higher, providing optimum display quality for practical use. If the retardation value is smaller than 365 nm or larger than 445 nm, the contrast will further decrease.
In the liquid crystal display device of the invention, it is preferable that the liquid crystal layer is formed as a super twisted nematic layer, the twist angle of liquid crystal molecules in the liquid crystal layer is selected to be 180xc2x0 or larger, and crossing angle xcex8 between the orientation direction of liquid crystal molecules lying closest to the reflective layer and the absorption axis of the polarization layer is selected to fall within a range of xe2x88x9210xc2x0 to +10xc2x0.
According to the invention, the liquid crystal display device is constructed as an STN liquid crystal display device that produces a display by utilizing birefringence. The polarization layer and the liquid crystal layer are designed so that the crossing angle xcex8 between the absorption axis of the polarization layer and the orientation direction of the liquid crystal molecules lying closest to the reflective layer is not less than xe2x88x9210xc2x0 but not larger than +10xc2x0. The reason for this is as follows. If the crossing angle xcex8 between the absorption axis and the orientation direction is outside the range of xe2x88x9210xc2x0 to +10xc2x0, color shifting when ON voltage is applied to the liquid crystal layer or when OFF voltage is applied becomes pronounced and the display quality falls short of the practical level. If the crossing angle xcex8 between the absorption axis and the orientation direction is set within the range of xe2x88x9210xc2x0 to +10xc2x0, the color shift occurring at the time of the ON voltage application, as well as at the time of the OFF voltage application, can be reduced to a level low enough for practical use. The reason for setting the tolerance range of xc2x110xc2x0 is that the optimum value somewhat varies depending on the crossing angle xcex94xcfx86 between the absorption axis of the polarization layer and the retardation axis of the phase retardation layer, the retardation value of the liquid crystal layer, and the retardation value of the phase retardation layer.
In the liquid crystal display device of the invention it is preferable that the phase retardation layer is a uniaxially oriented film.
According to the invention, a uniaxially oriented film is used as the phase retardation layer. This serves to reduce the cost of the phase retardation layer.
In the liquid crystal display device of the invention, it is preferable that the liquid crystal display device further comprises two substrates sandwiching the liquid crystal layer therebetween, and the reflective layer is disposed between the liquid crystal layer and either one of the two substrates.
According to the invention, the liquid crystal display device includes two substrates and the reflective layer is disposed between the liquid crystal layer and either one of the two substrates. With this arrangement, the liquid crystal display device can alleviate the overall reflectivity degradation of the liquid crystal display device caused by the presence of the substrate and reduce the parallax associated with the thickness of the substrate.
In the liquid crystal display device of the invention, it is preferable that the liquid crystal display device further comprises a scattering layer for scattering light, which is disposed on the side of the liquid crystal layer closest to the polarization layer, and the side of the reflective layer closest to the liquid crystal layer is planarized.
According to the invention, since the reflective layer in the liquid crystal display device is planarized, the reflective layer can also be used as an electrode. Furthermore, since the positive reflection component of the light reflected by the specular surface can be suitably scattered by the scattering layer in the viewing direction, the apparent lightness of the liquid crystal display device increases.
In the liquid crystal display device of the invention it is preferable that the phase retardation layer is disposed between the polarization layer and the liquid crystal layer, and the scattering layer is disposed between the phase retardation layer and the liquid crystal layer.
According to the invention, the polarization layer, the phase retardation layer, the scattering layer, and the liquid crystal layer are stacked in this order from the top to the bottom of the liquid crystal display device. In the liquid crystal display device thus constructed, since backscattering from the scattering layer is reduced, the display blurring of the liquid crystal display device caused by scattering can be alleviated.
In the liquid crystal display device of the invention, it is preferable that the reflective layer is a light-semitransmitting layer which reflects only a portion of incoming light and allows the remaining portion thereof to pass through.
According to the invention, the liquid crystal display device is constructed as a light-semitransmitting type liquid crystal display device which combines the characteristics of a reflective liquid crystal display device with the characteristics of a light-transmitting type liquid crystal display device. Accordingly, when there is no light entering the liquid crystal layer from the one side thereof, the liquid crystal display device can produce a display by utilizing light entering the liquid crystal layer from behind the reflector. For example, when a backlight source is mounted on the side of the reflective layer opposite from the liquid crystal layer, the liquid crystal display device can be used if no ambient light is available.
In the liquid crystal display device of the invention, it is preferable that the liquid crystal display device further comprises a circular polarization selecting layer for selectively transmitting only a circular polarization component of the incident light, which circular polarization selecting layer is disposed on the side of the liquid crystal layer closest to the reflective layer.
According to the invention, when the liquid crystal display device is of a light-semitransmitting type, the light entering the liquid crystal layer from the opposite side thereof is already confined to circular polarization by the circular polarization selecting layer. When producing a display utilizing light passed through the reflective layer, the light passing through the reflective layer must be optimized so that the incident light emerges as circularly polarized light from the side of the reflective layer closest to the liquid crystal layer, similarly to the case where the light reflected by the reflective layer is used for display. With this arrangement, not only when producing a display utilizing the light reflected by the reflective layer, but also when producing a display utilizing the light passed through the reflective layer, the liquid crystal display device can achieve the characteristics that increase the brightness of white display, improve the contrast, and render the white and black display states in achromatic colors.
In the liquid crystal display device of the invention, it is preferable that the circular polarization selecting layer comprises a quarter wave layer and a polarization layer for transmitting therethrough only a linearly polarized component of incident light and polarized in a predetermined direction, and the quarter wave layer is disposed between the polarization layer and the liquid crystal layer.
According to the invention, when the liquid crystal display device is of a light-semitransmitting type, the circular polarization selecting layer can reliably confine the incident light to circular polarization before the light is passed through the reflective layer and enters the liquid crystal layer from the opposite side thereof. As a result, the light that enters the liquid crystal layer by passing through the reflective layer can be reliably confined to a polarization state close to circular polarization.
In the liquid crystal display device of the invention, it is preferable that the circular polarization selecting layer is a circular polarization selective reflecting layer which reflects either a right-hand circularly polarized component or a left-hand circularly polarized component of the incident light and allows the other component to pass therethrough.
According to the invention, the circular polarization selecting layer decomposes the incident light into the right-hand circularly polarized component and the left-hand circularly polarized component, and reflects either one of the components, while allowing the other component to pass therethrough; accordingly, when the liquid crystal display device is of a light-semitransmitting type, the light that enters the liquid crystal layer by passing through the reflective layer can be reliably confined to circular polarization.